Conventionally, signal processing devices for processing reception signals received by radar apparatuses (object detectors) which detect existence of objects on the sea, have been known. Generally, a signal processing device of a radar apparatus receives reflection waves caused by electromagnetic waves transmitted from the radar apparatus and suitably processes them to detect positions of objects on the sea (e.g., ships traveling on the sea).
Meanwhile, the ships traveling on the sea include a towing ship and a towed ship connected with each other by a towing line, such as a rope. The towed ship can travel on the sea by the towing ship pulling it via the towing line. For example, JP2007-083976A discloses a sea transportation monitoring system for monitoring a fixed state of cargo installed on a transportation barge (towed ship) towed by a tugboat (towing ship).
Regarding traveling of ships, if a ship intends to pass between a towing ship and a towed ship without noticing that they are in the relation between the towing ship and the towed ship as described above, the ship may be caught by the towing line. Although it can be considered to detect the towing line itself by a radar apparatus for prevention, since a towing line is much thinner than a ship, the detection thereof is extremely difficult.